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Abstract:

In a method of producing a metal structure by photoreducing metal ion, a
substance capable of suppressing growth of metal crystal is added to a
medium in which metal ion is dispersed to prevent growth of the metal
crystal produced by photoreduction of the metal ion, thereby processing
resolution of a metal structure formed of the metal crystal is improved.

Claims:

1. A method of producing a metal structure composed of metal crystal,
comprising a step of irradiating a medium containing metal ion dispersed
therein with light, to thereby photoreduce the metal ion to produce metal
crystal, wherein the medium contains a substance which blocks growth of
the metal crystal, wherein the light is a femtosecond short pulse laser
beam, and wherein the substance which blocks growth of the metal crystal
is Sodium N-decanoyl sarcosinate (NDSS).

2. (canceled)

3. (canceled)

4. (canceled)

5. (canceled)

6. (canceled)

7. (canceled)

8. (canceled)

9. (canceled)

10. (canceled)

11. The method according to claim 1, wherein the metal ion is a silver
ion.

12. The method according to claim 1, wherein the metal structure is a
linear structure.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention relates to a method of producing a metal
structure by photoreduction of metal ion. More specifically, the present
invention relates to a method of manufacturing a metal structure, whereby
processing resolution thereof is improved by suppressing growth of metal
crystal constituting the metal structure.

[0003] 2. Background Art

[0004] In recent years, fine processing technologies using light such as
optical lithography technology and optical disk manufacturing technology
are widely utilized, and such technologies have been studied in a variety
of fields.

[0005] For example, the fine processing technology using light, which is
most widely applied at present, is the above-mentioned optical
lithography technology. The optical lithography technology is a backbone
technology for manufacturing a variety of electronic devices such as
semiconductor chips. The technology relates to amassive copying
technology using a photo-transferring technology in principle, in which a
metal in a specified region is dissolved, separated out, or removed
finally in a chemical manner, thereby a desired metal pattern is formed
as a metal structure. Therefore, this method may be used only for
two-dimensional processing, and it is impossible to use the method to
freely form a metal structure having a three-dimensional structure.

[0006] On the other hand, a technique for forming a metal pattern by
irradiating directly a laser beam on a specified material is known as a
technology for forming a desired metal pattern as a metal structure other
than the above-mentioned optical lithography technology. More
specifically, there are a technique involving irradiating a focused laser
beam onto a medium having metal nanoparticles dispersed therein, to
thereby melt and bind the metal nanoparticles at the focal point of the
laser beam, thereby a metal pattern is formed as a metal structure; and a
technique involving irradiating focused light onto metal ion, thereby the
metal ion is photoreduced and a metal body is separated out, thereby a
metal pattern is formed as a metal structure.

[0007] Here, in the above-mentioned technique involving separating out the
metal body by photoreducing the metal ion, an arbitrary metal pattern can
be formed as a metal structure in response to the track of scanned
focused laser beam by scanning the focused laser beam which irradiates
the material ion. Accordingly, a metal structure having a
three-dimensional structure can be freely formed, and its applicable
range is extremely wide, thereby studies and developments have been made
upon the technique in a variety of fields in recent years.

[0008] A method of improving the processing resolution of the metal
structure in the technique involving photoreducing the metal ion to
separate out the metal body is also known.

[0009] In general, an absorption rate of light increases when metal
structure is separated out, and hence there are many cases where the
reaction proceeds explosively just at the moment when an increased amount
of the metal structure exceeds a certain threshold value. There is a
problem that, when such a phenomenon occurs, the photoreduction of the
metal ions existing in the vicinity of the focal point proceeds at the
same time, thereby the processing resolution of the metal structure
degrades. In the method disclosed in JP2006-316311A, a specified pigment
is added to a material, thereby an absorption spectrum and an absorption
cross section of a non-processed material are maintained at constant, and
the processing resolution is prevented from degradation by propagating
energy of the laser beam to an area other than the focal point of the
laser beam, besides photoreduction efficiency at the focal point of the
laser beam is improved.

[0010] This method could improve the processing resolution to a micrometer
order, but a higher-precision nanometer-order processing resolution was
yet required.

SUMMARY OF INVENTION

[0011] An object of the present invention is to provide a method of
producing a metal structure by photoreduction of metal ion, whereby
processing resolution is significantly improved compared with
conventional techniques. More specifically, an object of the present
invention is to provide a method of producing a metal structure, whereby
processing resolution thereof is improvedby suppressing growth of metal
crystal constituting the metal structure.

[0012] The inventors of the present invention have found that metal
crystal produced by photoreduction continues to grow for a while even
after light irradiation is stopped, and grows into a several micrometer
size, and that such a phenomenon restricts the processing resolution of
the metal structure produced by photoreduction of metal ion. Then, the
inventors have made extensive studies, and as a result, they have found
that a substance which can suppress growth of metal crystal when the
substance is contained in a medium in which metal ion is dispersed,
thereby completed the present invention.

[0013] That is, the present invention is as follows.

[1] A method of producing a metal structure composed of metal crystal,
comprising a step of irradiating a medium containing metal ion dispersed
therein with light, to thereby photoreduce the metal ion to produce metal
crystal, wherein the medium contains a substance which blocks growth of
the metal crystal. [2] The method according to [1], wherein the substance
has one or more functional groups selected from the group consisting of
ionic functional groups and coordinating functional groups. [3] The
method according to [2], wherein the substance having the ionic
functional group is represented by the general formula (I) or a salt
thereof:

R1--COOH (I)

[0014] In the general formula (I), R1 represents a saturated or
unsaturated hydrocarbon group in which any hydrogen atom may be replaced
by one or more substituents selected from the group consisting of
carboxyl, amino, thiol, hydroxyl and cyano groups, and any --CH2--
may be replaced by --C(═O)-- or --N(R2)--, and R2
represents an alkyl group.

[4] The method according to [2], wherein the substance having the ionic
functional group is represented by the general formula (II) or a salt
thereof:

R1--NH2 (II)

In the general formula (II), R1 represents a saturated or
unsaturated hydrocarbon group in which any hydrogen atom may be replaced
by one or more substituents selected from the group consisting of
carboxyl, amino, thiol, hydroxyl and cyano groups, and any --CH2--
may be replaced by --C(═O)-- or --N(R2)--, and R2
represents an alkyl group. [5] The method according to [2], wherein the
substance having the coordinating functional group is represented by the
general formula (III) or a salt thereof:

R1--SH (III)

In the general formula (III), R1 represents a saturated or
unsaturated hydrocarbon group in which any hydrogen atom may be replaced
by one or more substituents selected from the group consisting of
carboxyl, amino, thiol, hydroxyl and cyano groups, and any --CH2--
may be replaced by --C(═O)-- or --N(R2)--, and R2
represents an alkyl group. [6] The method according to [2], wherein the
substance having the coordinating functional group is represented by the
general formula (IV) or a salt thereof:

R1--OH (IV)

In the general formula (IV), R1 represents a saturated or
unsaturated hydrocarbon group in which any hydrogen atom may be replaced
by one or more substituents selected from the group consisting of
carboxyl, amino, thiol, hydroxyl and cyano groups, and any --CH2--
may be replaced by --C(═O)-- or --N(R2)--, and R2
represents an alkyl group. [7] The method according to [2], wherein the
substance having the coordinating functional group is represented by the
general formula (V) or a salt thereof:

R1--CN (V)

In the general formula (V), R1 represents a saturated or unsaturated
hydrocarbon group in which any hydrogen atom may be replaced by one or
more substituents selected from the group consisting of carboxyl, amino,
thiol, hydroxyl and cyano groups, and any --CH2-- may be replaced by
--C(═O)-- or --N(R2)--, and R2 represents an alkyl group.
[8] The method according to [1], wherein the substance is represented by
the general formula (VI) or a salt thereof:

R1--O--R3 (VI)

In the general formula (VI), R1 and R3 each represents a
saturated or unsaturated hydrocarbon group in which any hydrogen atom may
be replaced by one ormore substituents selected from the group consisting
of carboxyl, amino, thiol hydroxyl and cyano groups, and any --CH2--
may be replaced by --C(═O)-- or --N(R2)--, and R2
represents an alkyl group. [9] The method according to [1], wherein the
substance is represented by the general formula (VII) or a salt thereof:

R1--C(═O)--NH--R3 (VII)

In the general formula (VII), R1 and R3 each represents a
saturated or unsaturated hydrocarbon group in which any hydrogen atom may
be replaced by one or more substituents selected from the group
consisting of carboxyl, amino, thiol, hydroxyl and cyano groups, and any
--CH2-- may be replaced by --C(═O)-- or --N(R2)--, and
R2 represents an alkyl group. [10] The method according to [1],
wherein the substance is a polymer or a copolymer composed of a monomer
having one or more functional groups selected from the group consisting
of amino, carboxyl, carbonyl and thiol groups. [11] The method according
to any one of [1], wherein the metal ion is a silver ion.

BRIEF DESCRIPTION OF DRAWINGS

[0015] FIG. 1 is a drawing illustrating the action of a substance which
blocks growth of a silver crystal.

[0016] FIG. 2 is an electron micrograph of a silver line obtained by
adding NDSS as the substance which blocks growth of metal crystal to a
medium.

[0017] FIG. 3 is an electron micrograph of a silver line obtained by
adding no substance which blocks growth of metal crystal to a medium.

[0018] FIG. 4 is an electron micrograph of silver lines obtained by adding
DL-alanine as the substance which blocks growth of metal crystal to a
medium.

[0019] FIG. 5 is an electron micrograph of a silver line obtained by
adding DL-alanine as the substance which blocks growth of metal crystal
to a medium (enlarged micrograph of FIG. 4).

[0020] FIG. 6 is an electron micrograph of silver lines obtained by adding
sodium decanoate as the substance which blocks growth of metal crystal to
a medium.

[0021] FIG. 7 is an electron micrograph of silver lines obtained by adding
disodium sebacate as the substance which blocks growth of metal crystal
to a medium.

[0022] FIG. 8 is an electron micrograph of silver lines obtained by adding
sodium laurate as the substance which blocks growth of metal crystal to a
medium.

[0023] FIG. 9 is an electron micrograph of silver lines obtained by adding
DL-2-amino-n-octanoic acid as the substance which blocks growth of metal
crystal to a medium.

[0024] FIG. 10 is an electron micrograph of silver lines obtained by
adding sodium N-lauroyl sarcosinate hydrate as the substance which blocks
growth of metal crystal to a medium.

[0025] FIG. 11 is an electron micrograph of silver lines obtained by
adding poly(vinylpyrrolidone) as the substance which blocks growth of
metal crystal to a medium.

[0026] FIG. 12 is an electron micrograph of a silver rod formed by adding
NDSS as the substance which blocks growth of metal crystal to a medium.

[0027] FIG. 13 is an electron micrograph of a silver rod formed by adding
NDSS as the substance which blocks growth of metal crystal to a medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Hereinafter, embodiments of the present invention are described in
detail.

[0029] In the present invention, the "metal structure" refers to a
structure formed of metal crystals produced by photoreduction of metal
ion. Therefore, as the size of each metal crystal becomes smaller, the
processing resolution of the metal structure may be improved. The shape
of the metal structure manufactured by the method of the present
invention includes a one-dimensional structure such as a line or curve, a
flat two-dimensional structure, and a spatial three-dimensional
structure. According to the method of the present invention, a metal
structure can be formed in an arbitrary pattern at any part in a medium.
For example, it is easy to produce the metal structure on a part or whole
of the surface of the medium or to form the metal structure at the inside
thereof.

[0030] The light irradiated in the method of the present invention
provides energy to reduce metal ion, therefore, the light to be
irradiated has a wavelength at which the metal ion has an absorption.
However, in the case where the medium contains a substance, which
converts the wavelength of the irradiated light into the wavelength at
which the metal ion has an absorption, the light to be irradiated is not
limited to light having the wavelength at which the metal ion has an
absorption. Examples of the light source include laser light sources,
light-emitting diodes, and lamps, and the laser light sources are
preferably used because high energy is required to photoreduce the metal
ion. Usually, the light to be irradiated is focused using a focus lens
and focused in a medium in which metal ion is dispersed. Focusing of the
light can increase the photon density at the focal point to a very high
level, and may localize light energy necessary for reduction of the metal
ion at the focal point. As a result, the metal ion can be photoreduced
only in the vicinity of the focal point, and a fine metal structure can
be manufactured along the track of the focal point by scanning the focal
point. Moreover, the light may be focused to perform fine processing at a
size scale of the focal point. In addition, femtosecond short pulse laser
beam, or the like may be used to provide higher light energy per unit
time.

[0031] The intensity of the irradiated light required for photoreducing
metal ion depends on the type of the metal ion or the absorbance of the
metal ion at the wavelength of the irradiated light. That is, in the case
where the absorbance of the metal ion at the wavelength of the irradiated
light is low, the intensity of the irradiated light required for
photoreducing the metal ion is relatively high, while in the case where
the absorbance of the metal ion at the wavelength of the irradiated light
is high, the intensity of the irradiated light required for photoreducing
the metal ion is relatively low. Meanwhile, in the case where the medium
contains a substance which absorbs or scatters the irradiated light, the
intensity of the irradiated light becomes smaller as the irradiated light
passes through the medium. Therefore, it is necessary to relatively
increase the intensity of the irradiated light. Thus, the intensity of
the irradiated light necessary for photoreducing the metal ion may not be
simply determined, but may be adjusted to a range of, for example, about
0.1 mW to 10 mW before incidence to the medium. It is not necessary to
maintain the intensity of the irradiated light to a constant level during
processing, and the intensity may appropriately be changed.

[0032] As well as the intensity of the irradiated light, the scan rate of
the focal point may appropriately be adjusted depending on the absorbance
of the metal ion at the wavelength of the irradiated light or the
presence of a substance in the medium. Therefore, the scan rate necessary
for photoreducing the metal ion may not be simply determined, but may be
adjusted to a range of, for example, about 0.1 μm/s to 100 μm/s. It
is not necessary to maintain the scan rate to a constant level during
processing, and the rate may appropriately be changed.

[0033] Scanning of the focal point may be performed by, for example,
irradiating light to a medium, in which metal ion is dispersed, and which
is placed on a XYZ-axis stage, while the stage is moved
one-dimensionally, two-dimensionally, and three-dimensionally. The focal
point in the medium can be arbitrarily scanned by moving the stage. Also,
the scanning of the focal point may also be performed by arbitrarily
moving the position of the focal point in the medium, while the position
of the medium, in which the metal ion is dispersed, is fixed. The medium
in which the metal ion is dispersed and the focal point of the irradiated
light can be moved simultaneously for scanning the focal point.

[0034] To manufacture a metal structure having a three-dimensional
structure by scanning the focal point, it is preferred that metal
crystals produced in advance do not block the track of the focal point.
To achieve this, a metal structure could be manufactured by the
sequential reductions of the metal ions by continuously scanning the
focal point from a part, which is far from the incidence position of the
irradiated light to the medium, to a part near to the incidence position.
However, if the track of the focal point is not blocked, it is not
necessary to scan the light from the farthest part to the nearest part.
The focal point may be scanned in such a manner to manufacture a fine
metal structure having an arbitrary three-dimensional structure, and to
easily manufacture even a hollow metal structure such as a box.
Meanwhile, in the case where the track of the focal point may be blocked
by the metal crystals produced in advance, there may be employed a method
involving: vanishing the irradiated light; appropriately changing the
incidence angle of the irradiated light to the medium; and starting
irradiation from a position not to block the track of the focal point to
scan the focal point again.

[0035] In the present invention, as the metal ions to be photoreduced, the
following are exemplified.

[0036] The state in which metal ion to be photoreduced is dispersed in a
medium includes, for example, a state in which metal ion is dissolved in
an aqueous medium and a state in which metal ion is dispersed in a medium
such as an organic solvent or resin. The state in which metal ion is
dispersed includes a state in which the ion is dispersed as a form of
colloid or micelle.

[0037] In the present invention, the concentration of the metal ion
dispersed in a medium is not particularly limited, and is preferably in
the range of 0.001 M to 10 M. The concentration of the metal ion is more
preferably in the range of 0.01 to 1M.

[0038] The medium which can be used in the present invention is not
particularly limited as long as metal ion can be dispersed therein, and
the medium includes: liquids or fluids such as water, organic solvents,
and fats and oils; semisolids such as gel; and solids such as a resin
(which is preferably a substance soluble in an organic solvent such as
PMMA or PVA, or water), amorphous materials such as glass, inorganic
crystals which may be doped with metal ion, such as lithium niobate, and
the medium is preferably water. The medium in which metal ion is
dispersed may be directly irradiated with light, or may be placed in a
container or placed on a substrate and then irradiated with light through
the container or substrate. In the case where the medium in which metal
ion is dispersed is a liquid or fluid, light may be convergently
irradiated on a contact surface between the container and the medium or
between the substrate and the medium to form a metal structure on the
inner surface of the container or on the substrate.

[0039] In the present invention, the "substance which blocks growth of
metal crystal" refers to a substance which prevents crystals formed by
separating out of a metal from binding together and blocking the metal
crystals from becoming larger. Examples of such a substance include a
substance having an effect of covering the surface of metal crystal to
prevent binding of the metal crystal to another metal crystal (FIG. 1).
Based on this perspective, the substance which blocks growth of metal
crystal preferably has all of the following properties.

(1) To have an atom which has affinity for or binds directly to a metal
or metal ion in its molecule. (2) To be dispersed in a solvent in which
metal ion is dispersed. (3) To have a low ability to directly reduce
metal ion. (4) To form no precipitates by binding to metal ion. However,
if the precipitates can be dissolved again by pH adjustment of the
solvent or by formation of complex ion of metal ion, such a substance may
be used.

[0040] Of those substances, which are considered to satisfy the
above-mentioned conditions, the substance which can be used in the
present invention is a hydrocarbon chain further having at least one of
the following properties.

(5) To have an ionic functional group in its molecular structure. (6) To
have a coordination linkage functional group having an unshared electron
pair (lone pair) in its molecular structure. (7) To have a peptide bond
or a similar structure thereof, an ether bond, or an ester bond in its
molecular structure. (8) To have a carbonyl group in its molecular
structure.

[0041] The above-mentioned hydrocarbon chain includes a saturated or
unsaturated hydrocarbon chain and preferably includes a saturated
hydrocarbon chain, because the saturated hydrocarbon has a lower ability
to reduce metal ion.

[0042] The chain length of the hydrocarbon is not particularly limited and
may appropriately be selected so that the hydrocarbon can be easily
dispersed in a medium in consideration of the type of the medium to be
used or the presence of a hydrophilic group in the molecular structure.
In the case where water is used as a solvent and the molecular chain
includes no hydrophilic group, the length of the carbon chain is
preferably about 5 to 10.

[0043] The above-mentioned ionic functional group refers to a functional
group which can be ionized in an aqueous solution and includes anionic
and cationic functional groups. Examples of the anionic functional groups
include carboxyl, sulfonyl, phosphate, and silanol groups and salts
thereof; while examples of the cationic functional groups include amino
and pyridinium groups and salts thereof. Among the salts of the ionic
functional groups, the salts of the anionic functional groups include
sodium and potassium salts; while the salts of the cationic functional
groups include halogenated salts.

[0044] Examples of the above-mentioned coordination linkage functional
groups include thiol, hydroxyl, and cyano groups. The thiol group can
form a thiol bond together with gold, silver, copper, or the like.

[0045] A substance having the above-mentioned properties has affinity for
a metal or binds directly to a metal atom, and hence the substance can
coat the surface of the metal to prevent growth of metal crystal.

[0046] Substances represented by the following general formulae (I to VII)
and salts thereof are included in the substance which blocks growth of
metal crystal according to the present invention.

R1--COOH (I)

R1--NH2 (II)

R1--SH (III)

R1--OH (IV)

R1--CN (V)

R1--O--R3 (VI)

R1--C(=0)-NH--R3 (VII)

In the formulae, R1 and R3 represent a saturated or unsaturated
hydrocarbon group in which any hydrogen atom may be replaced by one or
more substituents selected from the group consisting of carboxyl, amino,
thiol, hydroxyl and cyano groups, and any --CH2-- may be replaced by
--C(═O)-- or --N(R2)--, and R2 represents an alkyl group.

[0047] The chain length of the alkyl group represented by R2 is not
particularly limited and may appropriately be selected in consideration
of the type of a medium to be used and the presence of a hydrophilic
group in the molecule structure. In the case where water is used as a
solvent and the molecular chain includes no hydrophilic group, the length
of the carbon chain is preferably about 5 to 10.

[0048] Examples of the salts of the substances represented by the above
general formulae (I to VII) include substances having a sodium salt of a
carboxyl group (--COONa), a potassium salt of a carboxyl group (--COOK),
a calcium salt of a carboxyl group ((--COO)2Ca), or a silver salt of
a carboxyl group (--COOAg). In addition, the substances represented by
the above general formulae (I to VII) or salts thereof include substances
obtained by ionizing such substances, and the substance which blocks
growth of metal crystal according to the present invention includes a
substance having, for example, --COO.sup.- (produced by ionizing a
carboxyl group or a salt thereof), --NH3.sup.+ (produced by ionizing
an amino group), or --SO2.sup.- (produced by ionizing a sulfonic
group).

[0050] Specific examples of the substance represented by the above general
formula (II) and the salts thereof include amines such as 1-butyl amine
and 1-hexyl amine.

[0051] Specific examples of the substance represented by the above general
formula (III) and the salts thereof include thiols such as 1-butane thiol
and 2-aminoethane thiol.

[0052] Specific examples of the substance represented by the above general
formula (IV) and the salts thereof include some alcohols such as
6-amino-1-propanol and butanol.

[0053] Specific examples of the substance represented by the above general
formula (V) and the salts thereof include butyronitorile.

[0054] Specific examples of the substance represented by the above general
formula (VI) and the salts thereof include a substance having an ether
bond such as 3,3'-oxydipropionitorile.

[0055] Specific examples of the substance represented by the above general
formula (VII) and the salts thereof include peptides such as a dimer of
alanine.

[0056] In addition, a polymer or copolymer formed of a monomer having one
or more functional groups selected from amino, carboxyl, carbonyl, thiol,
hydroxyl, and cyano groups is also included in the substance which blocks
growth of metal crystal according to the present invention. Specific
examples of the substance include poly(vinylpyrrolidone). The molecular
weight of the polymer or copolymer is preferably about 40,000 to 80,000.

[0057] The concentration of the substance which blocks growth of metal
crystal in a medium is not particularly limited and is preferably in the
range of 0.001M to 10 M. The concentration is more preferably in the
range of 0.01M to 1 M.

[0058] The temperature at which a metal structure is produced in the
present invention is not particularly limited but is preferably in a
temperature range in which the medium can maintain its original
properties. For example, in the case where water is used as a medium,
water freezes at a temperature below zero and evaporates at too high
temperature. Therefore, it is impossible to maintain the original
properties of water, and metal ion is reduced only at a high temperature,
which is not preferable. In such case, the reaction temperature is
preferably about 5 to 60° C., and usually, processing can be
performed at room temperature. If optical energy is absorbed by metal
ion, or the like, the energy is converted into heat, which may cause an
increase in the temperature of the medium during processing. Therefore,
if necessary, a cooling apparatus may be used to suppress an increase in
the temperature.

EXAMPLES

[0059] Hereinafter, the present invention will be explained in more detail
in examples described below. However, the scope of the present invention
is not limited to the examples.

Example 1

Shape of Metal Structure Obtained by Adding a Substance which Blocks
Growth of Metal Crystal to the Medium

[0060] Sodium N-decanoyl sarcosinate (NDSS) (formula VIII) was added to an
aqueous solution of silver nitrate, and the solution was dropped on a
glass substrate (Micro Cover Glass, manufactured by Matsunami Glass Ind.,
Ltd.). Laser beam (light source: Titanium:sapphire femtosecond laser
(Tsunami (registered trademark), manufactured by Spectra-Physics K.K.),
center wavelength: 800 nm, pulse width: 80 fsec), which was controlled to
be focused on the upper surface of the glass substrate, was irradiated
from the bottom of the glass substrate and the focal point was scanned
linearly in a horizontal direction to the surface of the substrate. The
final concentration of NDSS was 0.1 M, the concentration of silver
nitrate was 0.05 M, the strength of the irradiated laser beam was 0.8 mW,
and the scan rate was 7 μm/s. Processing was performed at room
temperature.

CH3(CH2)8--CO--N(CH3)--CH2--COONa (VIII)

NDSS

Comparative Example

Shape of Metal Structure Obtained by Adding No Substance which Blocks
Growth of Metal Crystal to the Medium

[0061] A solution obtained by adding a solution of Coumarin 400
(manufactured by Exciton, purchased from Tokyo Instruments, Inc.) in 0.01
wt % ethanol to an aqueous solution of silver nitrate was dropped on a
glass substrate (Micro Cover Glass, manufactured by Matsunami Glass Ind.,
Ltd.). Laser beam (light source: Titanium:sapphire femtosecond laser
(Tsunami (registered trademark), manufactured by Spectra-Physics K.K.),
center wavelength: 800 nm, pulse width: 80 fsec), which was controlled to
be focused on the upper surface of the glass substrate, was irradiated
from the bottom of the glass substrate, and the focal point was scanned
linearly in a horizontal direction to the surface of the substrate. The
concentration of silver nitrate was 0.05 M, the strength of the
irradiated laser beam was 0.8 mW, and the scan rate was 7 μm/s.
Processing was performed at room temperature.

[0062] FIG. 2 shows an electron micrograph of a linear silver structure
(silver line) formed of silver crystals formed on the substrate by the
method of Example 1, and FIG. 3 shows an electron micrograph of a silver
line formed on the substrate by the method of Comparative Example. FIG. 2
indicates that the formed silver line has a width of about 150 nm and
that the particle of each silver crystal in the silver line has a
nanometer size. On the other hand, FIG. 3 indicates that the silver line
has a width of about 1 μm and that the surface is bumpy because of
large rocky aggregates formed by growth of small particulate silver
crystals. If such large aggregates are present, it is difficult to form a
line with a smaller width by the method of Comparative Example, because
the final line width depends on the sizes of the large particles.

[0063] Further it should be noted that, although the size of the laser
focal point is about 1 μm, a silver line with a width about ten times
smaller could be drawn by the method described in Example 1.

[0064] FIG. 2 indicates that the particle size of each silver crystal is
very small compared with the width of the silver line, which suggests
that it is possible to achieve finer processing if the spot size of the
focal point of the laser beam is controlled to be smaller or if light is
irradiated only to a smaller region.

Examples 2 to 8

[0065] The same experiments were performed using aqueous solution of
silver nitrate, to which various substances according to the present
invention (the following chemical formulae IX to XV) were added instead
of NDSS used in Example 1, to thereby form silver lines. The silver lines
thus formed are shown in FIGS. 4 to 11. Experimental conditions of
respective examples are shown in Table 1. Processing was performed at
room temperature.

[0074] In all the examples, the widths of the silver lines were in the
range of 200 to 300 nm, which are very small compared with the size in
the case of the comparative example. Meanwhile, also in the case where
sodium sorbate was added as an unsaturated hydrocarbon, a line was
formed. However, the metal ion was reduced directly by the sodium sorbate
with time, which suggests that, in the case where a substance containing
an unsaturated hydrocarbon is used, it was necessary to offset the
reduction ability of the unsaturated hydrocarbon by, for example, adding
an antioxidant immediately after processing, or previously adding an
antioxidant to a material containing the metal ion.

Example 9

Formation of Three-Dimensional Structure

[0075] A metal structure having a three-dimensional structure was formed
by the method of producing a metal structure of the present invention.

[0076] NDSS (Chemical formula V) was added to an aqueous solution of
silver nitrate, and the solution was dropped on a glass substrate (Micro
Cover Glass, manufactured by Matsunami Glass Ind., Ltd.). Laser beam
(light source: Titanium:sapphire laser (Tsunami (registered trademark),
manufactured by Spectra-Physics K.K.), center wavelength: 800 nm, pulse
width: 80 fsec), which was controlled to be focused on the upper surface
of the glass substrate (spot size of focal point is about 1 μm), was
irradiated through the glass substrate from the bottom of the glass
substrate, and the focal point was scanned linearly in a perpendicular
direction to the surface of the substrate. The final concentration of
NDSS was 0.1M, the concentration of silver nitrate was 0.04 M, the
strength of the irradiated laser beam was 1.21 mW, and the scan rate was
2 μm/s (first round) and 4 μm/s (second round).

[0077] FIG. 12 shows an electron micrograph taken from obliquely above of
a cylindrical silver structure (silver rod) which is formed of silver
crystals formed on a substrate under the condition at the first round and
stands upright on the substrate. This confirms that a silver rod having a
cross section diameter of about 300 nm and having a smooth surface was
formed. The magnified micrograph of a silver rod obtained by processing
under the conditions at the second round (FIG. 13) shows that a silver
rod with the finest width of about 100 nm was successfully processed.

[0078] The result demonstrated that, according to the present invention, a
fine metal structure having a three-dimensional structure can be easily
formed only by controlling the direction of light scanning.

INDUSTRIAL APPLICABILITY

[0079] According to the present invention, the particle size of metal
crystal produced by photoreduction of metal ion can be controlled to a
nanometer size, thereby the method significantly improves the processing
resolution of a metal structure formed of the metal crystal. Therefore,
an arbitrary pattern of fine and precise three-dimensional structure of a
metal structure can be formed easily. The present invention can be used
for manufacture of amicromachine, formation of a magnetic field in a
microspace by forming of a small metal coil, or control of the refractive
index.

[0080] While the invention has been described in detail with reference to
preferred embodiments thereof, it will be apparent to one skilled in the
art that various changes can be made, and equivalents employed, without
departing from the scope of the invention. Each of the aforementioned
documents including the priority application JP2008-077913 is
incorporated by reference herein in its entirety.